Optoelectronic devices such as semiconductor lasers, amplifiers, photonic integrated circuits (PICS), and detectors suitable for generating or processing optical and electronic signals are typically grown on wafers of III-V materials such as InP or GaAs. Following growth and fabrication, the wafer is cut into separate devices known as chips or die which are then packaged into an operational configuration. The die need to be subjected to a heat sinking process in order to provide stable optoelectronic properties and high levels of performance. In particular, there is a critical need for providing a non-destructive, clean, stable, thermally conductive bond between optoelectronic devices and their heat sinks. This bond is known as the diebond.
Diebonding employs a heat spreader made of a material with high thermal conductivity such as diamond, cubic boron nitride, or copper. Diebonds for optoelectronics are usually made with solder, although either solder or conductive plastics can be used for electronic diebonds. Diebonding of optoelectronics is difficult since the devices require exceptionally high quality heat sinking to provide functionality, are exceptionally small in size compared to pure electrical devices, must be kept clean to maintain optical performance, and are easily damaged.
Optoelectronic devices relying upon exposed structures such as ridge waveguides to optically guide light, or that are grown on soft, brittle wafers such as InP, are easily damaged during component packaging. For example, exposed structures would be pressed into the solder or contacted by a vacuum pick-up tool during conventional diebonding, depending upon chip orientation. One particularly good solder for diebonding optoelectronics is the eutectic alloy of gold-tin (80/20 ratio). This alloy melts at 280.degree. C. and can be used without flux to provide for an optically clean process.
However, one significant drawback of the gold-tin solder is that it tends to destroy optoelectronic ridge devices diebonded in the epi-down geometry. The probable mechanism responsible for the destruction of the device is that the hard gold-tin solder "crushes" the InP ridge structures during the temperature excursion of diebonding, thus electrically shorting the devices. Destruction does not occur, however, if the devices are bonded epi-up so that the device structure is placed away from the solder. However, epi-down geometry is preferable since it places the source of heat production, namely the device structure, in closest proximity to the heat sink. This particular packaging orientation also permits certain mechanical packaging options that rely upon precise measurements of the distance between the optical region of the device and its bonded surface. This distance is most easily determined for the epi side because the gross thickness of the die need not be considered.